I think the increase in allergies is due to children not being exposed to allergens. It used to be the advice to not feed your child foods that are commonly allergenic but now the advice is reversed.
I grew up on an acreage, drinking well water in the 80s - I doubt it was contaminated by micro-plastics. We had animals of all sorts. I still have brutal allergies - the irony being that the things I was exposed to the most (we kept horses) is the thing I'm most allergic to.
Pregnant woman feeds in such a way that develops propensity to diabetes in the baby.
Certain exposures to food, bacteria, fungi, protozoa make us hypersensitive to these or other microbes over time.
Your situation with the horses is curious. It may or may not be caused by genetics, I imagine you have already thought about it a lot.
There are unknowns for autism, high cancer rates, autoimmune disorders, near-sightedness